This application claims priority of Japanese Application No. 2000-349990, filed Nov. 16, 2000, the complete disclosure of which is hereby incorporated by reference.
a) Field of the Invention
The present invention relates to a polygonal mirror fixing device in which a polygonal mirror placed over a rotary body is fixed by the press-fitting of a mirror fixing member via a spring member.
b) Description of the Related Art
In general, a polygonal mirror fixing device, in which a polygonal mirror is placed around a rotary body, is formed integral with a motor 1, as illustrated in FIGS. 2 and 3. A polygonal mirror 4 is axially placed over a rotary seat 3 of a cylindrical shape with a step, which is a part of a rotor 2 of the motor 1. More specifically, a center opening of the polygonal mirror 4 is axially placed over a mounting circumference 3a of the rotary seat 3; in the figure, the bottom end of the polygonal mirror 4 contacts a mounting bottom surface 3b which extends in the radially outward direction from the mounting circumference 3a. 
In the figure, an outer periphery of a mirror fixing member 5 attached and fixed to the mounting circumference 3a of the rotary seat 3 is pressed toward the top end surface of the polygonal mirror 4 via a spring member 6. With the pressing force of the mirror fixing member 5 and the spring member 6 in the axial direction, the polygonal mirror 4 is axially pressed toward the mounting bottom surface 3a, and whereby the polygonal mirror 4 is fixed.
Thus, the center opening of the polygonal mirror 4 is placed over the mounting circumference 3a of the rotary seat 3. A predetermined clearance is provided between the center opening of the polygonal mirror 4 and the mounting circumference 3a of the rotary seat 3 so that an excessive force will not be applied to deform the polygonal mirror 4 when the mounting circumference 3a of the rotary seat 3 passes through the center opening of the polygonal mirror 4. Therefore, the mirror fixing member 5 presses the polygonal mirror 4 toward the mounting bottom surface 3b of the rotary seat 3 with resilience via the spring member 6; by the pressing force and the friction caused due to the pressing, the polygonal mirror 4 is fixed.
However, while a ferrous material is generally used for the spring member 6, an aluminum material is generally used for the polygonal mirror 4. Thus, the polygonal mirror 4 is fixed as the members having different coefficients of linear expansion are in contact. Consequently, when the ambient temperature changes, the dimension is changed differently between the members 4 and 6, especially, in the radial direction. This causes a relative displacement in the radial direction between the members 4 and 6. Also the center of gravity of the polygonal mirror 4 is shifted from the center of rotation, and whereby rotational vibrations may be caused.
Accordingly, a primary object of the present invention is to provide a polygonal mirror fixing device of a simple configuration in which a displacement of the polygonal mirror that normally occurs due to the change in the ambient temperature can be prevented in order to obtain a steady rotation.
In accordance with the present invention, the above object is achieved by providing a polygonal mirror fixing device in which a rotary body to which a polygonal mirror is mounted has a mounting circumferential surface and a mounting bottom surface. The mounting circumferential surface is in a center opening of the polygonal mirror and said mounting bottom surface projecting from the mounting circumferential surface in the radially outward direction and having contact with an axial end surface of the polygonal mirror. A mirror fixing member and a spring member are attached axially from the outside of the polygonal mirror to the mounting circumferential surface of the rotary body, and by the pressing force of the mirror fixing member in the axial direction, the polygonal mirror is pressed in the axial direction toward the mounting bottom surface so that the polygonal mirror is fixed to the rotary body; wherein a mirror pressing member composed of the same material as the polygonal mirror is positioned such that it has contact with an axial end surface of the polygonal mirror, the other side from the end surface that contacts the mounting bottom surface; the mirror fixing member is attached axially to the outer side of the mirror pressing member to press the mirror pressing member in the axial direction; and by the pressing force of the mirror fixing member in the axial direction, the mirror pressing member is pressed in the axial direction toward the mounting bottom surface so that the polygonal mirror is fixed.
In other words, according to the polygonal mirror fixing member having the above configuration, the mirror pressing member that directly contacts the polygonal mirror is composed of the same material as the polygonal mirror. Therefore, the polygonal mirror is fixed such that the members having the same coefficient of linear expansion are in contact with each other. Consequently even when the ambient temperature changes, causing the diameter of the polygonal mirror to increase or decrease, there is no relative displacement in the radial direction between both members. Thus, the center of gravity of the polygonal mirror is always in accordance with the center of rotation to obtain a continuous, smooth rotation.
Also, since the mirror pressing member is fixed to the fitting surface of the mounting circumference of the rotary body in such a way as not to move at least in the radial direction, a displacement of the mirror pressing member in the radial direction is prevented from occurring.
Further, according to the polygonal mirror fixing member in accordance with the invention, the mirror fixing member is attached to the fitting surface and also positioned to contact the mirror pressing member via the spring member; by the pressing force of the mirror fixing member in the axial direction via the spring member, the mirror pressing member is pressed in the axial direction. In addition to the above effect, the polygonal mirror is pressed with resilience.
Further, according to the polygonal mirror fixing member in accordance with the invention, the outer diameter of the fitting portion which is provided at the rotary body for attaching the mirror pressing member is formed smaller than the outer diameter at the mirror mounting circumferential surface of the rotary body. Consequently the mirror fixing member which is attached to the fitting portion to press the mirror pressing member is not necessarily formed larger, but one having a smaller diameter can be used to give a resilient pressing effect. Thus, the entire device can be smaller at low cost.
Further, in the polygonal mirror fixing device of the present invention, the mirror pressing member is fixed by press-fitting to the fitting portion of the rotary body; thus, the mirror pressing member can be fixed very easily.
Further, in the polygonal mirror fixing device of the present invention, the mirror fixing member is coaxially fitted to the fitting portion of the rotary body from the axial direction. This makes the assembling process efficient.